There is known a fuel cell system that includes: a gas-liquid separator storing and separating water from a fuel gas discharged from a fuel cell; and a discharge valve connected to the gas-liquid separator and discharging the fuel gas to the outside together with the stored water in the gas-liquid separator. For example, Japanese Unexamined Patent Application Publication No. 2005-302708 describes technology for estimating a discharge amount of the fuel gas by opening the discharge valve.
The discharge amount of the fuel gas is estimated conceivably based on, for example, a differential pressure between upstream and downstream sides of the discharge valve during the period when the discharge valve is opened. However, the inventors found that the estimation of the discharge amount of the fuel gas by the above method might generate a large error between the estimated discharge amount and the actual discharge amount.